1. Field of The Invention
The present invention relates generally to a system and method for controlling damping force characteristics of respective shock absorbers of a vehicle, each shock absorber (also called a damper, but hereinafter referred to as the shock absorber) being interposed between an unsprung mass and sprung mass of the vehicle so as to provide an optimum damping force therefrom. The shock absorber is provided with varying means for varying the damping characteristic thereof at multiple stages in response to a drive signal.
2. Description of The Background Art
A Japanese Patent Application First Publication No. Showa 61-163011 published on Jul. 23, 1986 exemplifies a previously proposed damping characteristic controlling system for a vehicle.
In the disclosed Japanese Patent Application First Publication, a sprung mass vertical velocity and a relative velocity between sprung mass (vehicle body) and unsprung mass (tire wheel) are determined. When both velocities have mutually the same sign (both upward directions or both downward directions), the characteristics of the damping forces of the corresponding stroke side for any one or more shock absorbers are set to provide hard characteristics (HS or SH). When both velocities have the different signs (+ or -), the characteristics of the damping forces for the shock absorbers are set to provide soft characteristics. This setting operations are based on the damping force characteristic control called "Sky Hook" theory and carried out independently of four tire wheels.
That is to say, when, with the direction of the vertical velocity V being upward, the direction of the relative velocity Sv is switched from a contracted direction (minus) to an extended direction (plus), the damping characteristic is changed from a soft (low damping characteristic) to a hard (high damping characteristic). In an actual practice, a delay occurs at a rising of the damping force. Therefore, a lacking state of the damping force occurs at the initial stage of the switching of the damping characteristic. At the initial rising stage, the application of vibration to the vehicle body cannot sufficiently be suppressed.
In addition, when, with the direction of the relative velocity Sv in the extended direction (plus), the direction of the vertical sprung mass velocity is switched from the upward direction (plus) to the downward direction (minus), the damping characteristic is switched from the hard (high) damping characteristic to the soft (low) damping characteristic. However, since, in the actual practice, a response delay in the hydraulic (working fluid) in the shock absorber occurs, the damping force is not immediately reduced so that an excessive damping force occurs at the initial stage of switching of the damping force characteristic. Accordingly, the vibration suppression effect of the vehicle body cannot sufficiently be exhibited at this region of the initial stage of switching.
Furthermore, in the damping characteristic control based on the conventional "sky hook" theory, it is necessary to switch the damping characteristic by driving the actuator (namely, a pulse motor) whenever the coincidence or non-coincidence of both direction determining signs of the vertical sprung mass and vertical relative velocities is switched. Therefore, a control response characteristic becomes worsened and the number of times drives are provided to the actuators are increased, thereby the durability of the actuators being reduced.